I generally use a Cateye TL-LD155-R Omni 5 with rechargable AAA batteries. The mounting brackets are adjustable & easy to fit/remove.

I have 2 of them & interchange them between bikes or clothing, I also mount them my wedge bag using the clothing bracket.

Other than the rubber cap (over the switch going missing on one of them) they've never let me down. They have 3 settings, constant, random ish & flashing. The battery life seems pretty good to me but as I said I use rechargeable cells so charge them as & when necessary.

I also use Giant Numen Aero Tail Light which uses 2 x 10p sized CR2032 batteries as the fitting is more suited to a seat post fitting. These batteries are about £2 each & last about 80 hours in flashing mode. The light still works with only 1 battery in, I discovered by chance, so that's handy.